how do traffic lights work

Traffic lights work by using a combination of sensors, controllers, and signal heads to manage traffic flow safely and efficiently. The core technology involves detectors, often inductive loops embedded in the road surface, which sense the presence of vehicles by detecting changes in electromagnetic fields caused by metal in the vehicle. These detectors send information to a controller—essentially a computer—that decides when to change the lights based on programmed logic considering traffic demand, timing cycles, and safety requirements. Traffic lights typically operate in two main ways: fixed period and vehicle-triggered. Fixed period lights change signals according to preset times regardless of traffic, while vehicle-triggered lights adjust the timing based on real-time detection of vehicles waiting or passing through an intersection. This can include semi-triggered systems, which respond only to certain approaches, or fully triggered ones that adjust based on all traffic demands. The system also incorporates safety features like intergreen periods (all red lights) to ensure intersections clear before conflicting traffic moves. In sum, traffic lights are complex systems combining sensors (like inductive loops), programmable controllers, and signal lights to control traffic flows dynamically or by timer to ensure safety and reduce congestion on roads and intersections.